1. Field of the Invention
The present invention relates to a curtain coating apparatus and a curtain coating method, in which at least one layer of a coating liquid is ejected from a slit, the ejected coating liquid is made to fall freely by means of a pair of curtain edge guides that guide the coating liquid in the form of a curtain, and the coating liquid is thus applied onto a continuously running support.
2. Description of the Related Art
Conventionally, curtain coating methods have been proposed as coating methods for use in the production of photosensitive materials such as photographic films.
Examples of the curtain coating methods include (i) a method in which a coating liquid is ejected from a nozzle slit, made to fall freely by means of a pair of curtain edge guides (which guide the coating liquid in the form of a curtain) and brought into collision with a continuously running support (hereinafter, the term “support” will be referred to also as “web” or “base material”) so as to form a coating film; (ii) a method in which a coating liquid is ejected from a slit, moved on a slide surface, made to fall freely by means of a pair of curtain edge guides (which guide the coating liquid in the form of a curtain) and brought into collision with a continuously running web so as to form a coating film; (iii) a method (multilayered coating method) in which coating liquids with different compositions are ejected from respective nozzle slits, made to fall freely by means of a pair of curtain edge guides (which guide the coating liquids in the form of a curtain) and sequentially brought into collision with a continuously running web so as to form a coating film; and (iv) a method (multilayered coating method) in which coating liquids with different compositions are ejected from respective nozzle slits, layered over a slide surface, made to fall freely by means of a pair of curtain edge guides (which guide the coating liquids in the form of a curtain) and brought into collision with a continuously running web so as to form a coating film.
For instance, there has been proposed a method in which a coating liquid 3 is ejected from a nozzle slit of a slot curtain coating head 1, made to fall freely by means of a curtain edge guide 2 (which guides the coating liquid in the form of a curtain) and brought into collision with a continuously running web 5 so as to form a coating film, as shown in FIG. 9; and there has been proposed a method in which a coating liquid is ejected from a slit, moved on a slide surface 8 of a slide curtain coating head 7 (with an edge of the coating liquid being supported by a slide portion edge guide 9), made to fall freely by means of a curtain edge guide 2 (which guides the coating liquid in the form of a curtain) and brought into collision with a continuously running web 5 so as to form a coating film, as shown in FIG. 10 (refer, for example, to Japanese Patent Application Publication (JP-B) No. 49-35447). In FIGS. 9 and 10, there are respective vacuum devices provided.
Examples of multilayered coating methods include a method in which coating liquids with different functions are ejected from respective nozzle slits, made to fall freely by means of a pair of curtain edge guides (which guide the coating liquids in the form of a curtain) and brought into collision with a continuously running web so as to form a coating film; and a method in which coating liquids with different functions are ejected from respective slits, layered over a slide surface, made to fall freely by means of a pair of curtain edge guides (which guide the coating liquids in the form of a curtain) and brought into collision with a continuously running web so as to form a coating film.
In the case where coating liquid(s) is/are applied onto a web by a curtain coating method, instability of a freely falling curtain film has great adverse effects on productivity and product quality.
Typical examples of phenomena in which the stability of a curtain film is hindered include a phenomenon in which a curtain film shifts toward the back (hereinafter referred to as “teapot phenomenon”), and a phenomenon in which the thickness of a curtain film decreases in the vicinities of edge guide walls (refer, for example, to S. F. Kistler, and Schweize “Liquid Film Coating”).
The teapot phenomenon is a phenomenon in which a curtain film shifts toward the back of a lip instead of falling vertically. This is due to an imbalance in the momentum of a coating liquid (coating liquid flowing down a slide surface) at a lip edge.
The teapot phenomenon is particularly noticeable when the viscosity of a coating liquid decreases or the amount thereof applied increases, in other words when the Reynolds number is relatively large. Since both edges of the curtain film are supported by a pair of edge guides, arbitrary curving of the curtain film caused by the teapot phenomenon cannot be allowed, and thus the curtain film distorts.
Consequently, the amount of the coating liquid applied is uneven with respect to the width direction of the curtain film, and thus a favorable coating film cannot be obtained.
As a countermeasure against the teapot phenomenon, there has been proposed an edge guide which is curved so as to match the shape of a curtain film (refer, for example, to Japanese Patent Application Laid-Open (JP-A) No. 09-253552).
This proposal makes it possible to eradicate the distortion of a curtain film caused by the teapot phenomenon. However, since the extent of deformation of a curtain film stemming from the teapot phenomenon varies greatly depending upon operational conditions such as the properties of a coating liquid and the flow rate thereof, there is a problem in that it is necessary to change the shape of the edge guide according to the conditions, which is not satisfactory in practical use.
To solve this problem, there has been proposed a flat plate type edge guide, wherein a surface along which edge guide auxiliary water flows down (hereinafter referred to as “edge guide auxiliary water flow-down surface”, “auxiliary water flow-down surface” or “flow-down surface”) is formed as a flat plate so as to allow for the curtain film curvature that greatly varies depending upon the operational conditions, and the flow-down surface has a width which is sufficient for the deformation amount of the curtain film (refer, for example, to JP-A No. 2001-46939).
However, since the edge guide auxiliary water flow-down surface is a flat plate, the falling position of the curtain film on the edge guide varies due to slight airflow in the vicinity of a curtain coating apparatus or air which accompanies a web. If the variation is great, there is a problem in that the curtain film comes into contact with ends (with respect to the width direction) of the edge guide auxiliary water flow-down surface and so the coating film thickness becomes uneven.
Moreover, there may be a problem in that the falling position of the curtain film on the web varies and thus coating unevenness arises.
There has been proposed an edge guide in which an edge guide auxiliary water flow-down surface has a convex shape at the center with respect to the coating width direction (refer, for example, to International Publication No. WO2008/000507).
It has been confirmed that this proposal makes it possible to secure center adjustability of the edge guide and suppress the occurrence of coating unevenness that stems from the variation in the falling position of a curtain film caused by disturbance.
In this proposal, however, the convex shape of the edge guide auxiliary water flow-down surface has a constant curvature from its upper portion to its lower portion. Therefore, when the curvature of the curtain film related to the teapot phenomenon is great, a three-dimensional liquid flow is created, the curved portion deviates greatly from the apex of the convex shape of the flow-down surface, and consequently a coating liquid flows to a portion which is away from the apex of the convex shape of the edge guide.
The flow of the coating liquid to the portion away from the apex causes widening of the width of the curtain film, and both edges of the curtain film are pulled toward the center with respect to the curtain width direction owing to an increase in the surface tension of the coating liquid. Consequently, the curtain film shifts to the apex of the convex shape and falls down along the apex, and there is a problem in that the curvature of the three-dimensional liquid flow causes unevenness of the thickness of a coating film at its edges.
The phenomenon in which the thickness of a curtain film decreases arises in the vicinities of edge guides, notably anywhere at a distance of approximately several millimeters to approximately 10 mm from each edge guide. A result of research involved in the present invention has revealed that the thickness of the curtain film at such a portion is approximately 60% to approximately 95% of that of the curtain film at a central portion.
When the thin film portion has been applied onto a web, there is a thin film portion formed inside both edges of a coating film, which causes unevenness of coating film thickness and thus loss of coating.
Regarding the film thickness decrease phenomenon, development of a boundary layer, which is due to fluid friction between a free fall portion of the curtain film and the curtain film in the vicinity of each edge guide at the time of the fall of the curtain film, causes the curtain film forming coating liquid in the vicinity of each edge guide to shift toward the center with respect to the curtain width direction. Also, the difference in surface tension between a flow portion of the coating liquid in the vicinity of each edge guide and a steady flow portion of the coating liquid at the center with respect to the curtain width direction, which is due to dynamic properties of a surfactant in the coating liquid, similarly causes the curtain film forming coating liquid to shift toward the center with respect to the curtain width direction.
Meanwhile, it is known that a concave meniscus lying between the coating liquid at each edge guide and the gas phase causes the curtain film forming coating liquid to shift toward each edge guide (refer, for example, to J. Van Havenbergh, H. Bussmann, and P. Joos: Colloid Interface Sci., 101, 462, (1984)).
The unevenness of film thickness is suppressed by securing a favorable balance between the tendency for the curtain film forming coating liquid to shift toward the center with respect to the width direction (which stems from the development of the boundary layer and the difference in surface tension) and the tendency for the curtain film forming coating liquid to shift toward each edge guide (which stems from the concave meniscus) (refer, for example, to Japanese Patent (JP-B) No. 2630512).
As a means for achieving the foregoing, the level of the coating liquid viscosity, the difference in surface tension between a curtain film central portion and curtain film edges, and the dimensions of a liquid contact portion of each edge guide are defined so as to secure a favorable balance as described above.
This method enables uniformity of film thickness. In reality, however, in view of the product deign, it is difficult to limit the difference in surface tension when there is a formulation-related restriction, such as a restriction on properties of the coating liquid, provided to achieve high coating film quality. Thus, there is a problem in that the uniformity can be realized only under limited coating liquid conditions.
Also, a result of research involved in the present invention has revealed that a thick film portion exists on the inside (with respect to the curtain film width direction) of the thin film portion. However, a method for reducing the thick film portion has not hitherto been disclosed as opposed to methods relating to the thin film portion.
In the above-mentioned curtain coating methods, there is a phenomenon caused in which when the coating liquid falls freely, a portion (boundary layer) where the coating liquid flows slowly exists near each edge of the curtain film, and the difference in flow velocity causes the coating liquid near both edges of the curtain film to flow toward the center in a contracted manner. Thus, when the coating liquid is brought into collision with the continuously running web so as to form a coating film, there is a problem in that a thin film portion 120a (FIG. 11) forms near edges (with respect to the width direction) of a coating film 20 (FIG. 1) and a thick film portion 120b (FIG. 11) forms on an inner side (with respect to the width direction) of the coating film 20.
To prevent formation of the boundary layer in the curtain film, there has been proposed a technique in which by defining the viscosity and surface tension of the coating liquid and the shape of a liquid contact surface of each edge guide, formation of the boundary layer in the curtain film is suppressed, the formation of the thin film portion 120a and the thick film portion 120b caused by the flow of the coating liquid in a contracted manner is thereby prevented, and uniformity of coating film thickness is thus achieved (refer, for example, to JP-B No. 2630512).
However, this technique presents problems in that effects of the boundary layer can be lessened only under limited property conditions of the coating liquid and it is very difficult to regulate the viscosity and surface tension of the coating liquid.
Further, to prevent formation of the boundary layer in the curtain film, there has been proposed a technique in which by discharging an edge guide auxiliary liquid (in the direction in which a coating liquid flows down) to edge guides, formation of a boundary layer near each edge of a curtain film is prevented (refer, for example, to JP-A No. 01-199668).
However, this technique presents a problem in that the acceleration of the curtain film yielded by the edge guide auxiliary liquid does not suffice and thus formation of a boundary layer cannot be eradicated.
To stabilize the free fall of the coating liquid, there has been proposed a technique for exhibiting center adjustability of a curtain film, wherein an edge guide auxiliary liquid flow-down surface has an arc-like convex shape; thus, when there is no wind-based disturbance, a curtain film is positioned at the apex of the convex portion, and when the curtain film has deviated from the apex of the convex portion owing to wind-based disturbance, the deviating curtain film is returned to the apex of the convex portion by increasing the dynamic surface tension of the coating liquid (refer, for example, to 2008-529753).
However, this technique presents a problem in that when the static surface tension of the coating liquid is as small as approximately 35 mN/m, the curtain film deviates from the apex of the convex portion and adheres to a side surface of an edge guide owing to wind-based disturbance, thereby leading to unevenness of the curtain film. Also, this technique presents another problem in that as the coating liquid falls nonlinearly, the uniformity of a coating film is impaired, and coating unevenness arises. Further, this technique presents yet another problem in that a porous material for ejecting the edge guide auxiliary liquid is clogged with the coating liquid, thereby leading to uneven ejection of the edge guide auxiliary liquid.
If the coating liquid is attached to the porous material, it is washed off using a solvent such as hydrochloric acid. However, a difficult decomposing operation and the like are required and removal of the clogging is difficult, so that there is strong demand for development of clogging-free edge guides.
To solve the problem of clogging, there has been proposed a technique in which an edge guide auxiliary water flow-down surface is formed as a metal surface, and auxiliary water is ejected from an ejection port provided in the metal surface (refer, for example, to U.S. Pat. No. 7,081,163).
However, this technique relates to a structure in which the edge guide auxiliary water directly flows into the ejection port and thus presents a problem in that it is difficult to eject the edge guide auxiliary water uniformly, another problem in that since the edge guide auxiliary water flow-down surface is a flat surface, the auxiliary liquid does not fall linearly and so an unstable curtain film is formed, and yet another problem in that the curtain film does not shake owing to wind-based disturbance.